The invention relates to an economizer device for a refrigerating machine, a heat-pump or the like. The invention also relates to a machine equipped with such a device.
It is known to provide economizer devices in refrigerating systems or the like using rotating positive displacement or centrifugal multistage compressors. In such a system, as shown in FIG. 1, or in an alternative embodiment in FIG. 2, a compressor 1 takes in a refrigerant gas arriving through a conduit 2 and discharges it into a condenser 3 and a storage tank 4 in liquid form. In some units, as a practical matter, this tank is formed by a part of the condenser, but the tank will be maintained hereinbelow for a clear understanding of the description. From this tank the condensed liquid flows via a conduit 5 to a vaporization tank 6, the upper part of which is connected by a conduit 7 to at least one port 8 in the casing of the compressor 1 at a point where the pressure is intermediate between the intake pressure and the discharge pressure. Liquid is separated from the gas in this intermediate tank 6 and flows via a conduit 9 to an evaporator 10 after having travelled through an expansion valve 11. The gas vaporized in the valve 11 or the evaporator 10 returns to the compressor 1 via the conduit 2.
A valve 12 is mounted between the tanks 4 and 6 and is controlled by a float 13 measuring the level in the tank 6. In the same way, the valve 11 is controlled by a device 14 which measures the superheat at the evaporator exit. When more refrigeration is required from the evaporator 10, the device 14 opens the valve 11 wider, and the liquid level in the tank 6 decreases, whereby the opening of the valve 12 increases.
The known advantage of this economizer device is that a part of the gas formed to cool down the liquid going to the evaporator is recompressed from an intermediate pressure and not from the intake pressure. This improves the efficiency and increases the refrigerating capacity of the compressor. Nevertheless, this device has various drawbacks. First of all, it is bulky and expensive since it requires an extra tank 6 and an extra load of liquid refrigerant to fill up the tank. Moreover, the devices using floats are often subject to failures. Finally, this economizer device renders the system difficult to control because the expansion valve 11 does not work under the pressure existing between the condenser and the evaporator, but under the reduced difference between the intermediate pressure and the intake pressure, and because the system cannot work when the economizer is not itself in operation, for instance at part load of the compressor when said compressor is a screw compressor provided with slides. Instead, in this case, the pressure at the orifice 8 becomes equal to the intake pressure, and there is no longer a difference of pressure between the tank 6 and the evaporator 10 to permit the circulation of the liquid. Therefore, additional devices such as a check valve on the conduit 7 must be provided. However, such additional devices may then generate liquid bursts via the conduit 7 into the compressor upon reopening of said check valve, and the liquid which, when the valve was shut, was necessarily at the condenser pressure, suddenly tends to return to the intermediate pressure. Such bursts may to a certain extent cause serious damage to the compressor.
Thus, it is more usual to utilize the device represented in FIG. 2, wherein the conduit 9 and the expansion valve 11 are directly connected to the tank 4, but the device is provided with an exchanger 15 cooled by an auxiliary evaporator 16 placed on an economizer line 8 and fed by an expansion valve 17 controlled by superheat measuring means 18. In this embodiment, most of the drawbacks of the device of FIG. 1 are overcome since, whether the economizer is operating or not, the expansion valve 11 always works under the difference of pressure existing between the evaporator and the condenser. Nevertheless, this embodiment has other disadvantages. This device remains expensive because it requires an evaporator-exchanger and an additional expansion valve 17. On the other hand, the operation of the exchanger requires a temperature difference between the exchanger 15 and the auxiliary evaporator 16, said difference usually being of the order of 5.degree. C., the consequence being that the liquid reaching the valve 11 is much less supercooled than in the device shown in FIG. 1, and this substantially reduces the performance of the economizer and even eliminates any economizer performance at lower compression ratios.